The term “user equipment” (UE) is often used to refer to terminals, such as wireless connected handsets. In the context of the following discussion, machine-to-machine (M2M) terminal devices will be considered as UEs despite the fact that a user may not typically directly interact with them. An M2M network typically includes a large number of UEs that send relatively small amounts of traffic over wireless networks to other devices. For example, a UE installed on a vending machine can periodically report inventory to a centralized server operated by a supplier so that the supplier knows whether the vending machine needs to be restocked.
The UEs communicate with a base station using wireless communication resource blocks or slots (a “resource”). A resource may be shared in the frequency, time, code, and/or space dimension. As UEs proliferate, the amount of M2M traffic will increase. With the increase in M2M traffic, competition between UEs for resources will increase, resulting in congestion and access delays. Thus, a protocol that can more efficiently handle M2M traffic is needed.
More specifically, a conventional M2M network may use a dedicated gateway with custom MAC (media access control) protocols to access a wireless (e.g., cellular) network. The gateway is connected using cellular technology such as 3G (“third generation”) or LTE (Long Term Evolution).
However, it may not be practical to adapt a cellular network to schedule accesses or to manage resource reservation requests for M2M traffic. For one thing, the amount of overhead associated with scheduling or reserving resources may be too high, especially as the number of UEs in an M2M network continues to grow. For another, the process of scheduling or reserving resources may take too long to complete for more urgent M2M traffic, such as vehicle-to-infrastructure safety notifications.
A random access or contention-based protocol such as one based on IEEE (Institute of Electrical and Electronics Engineers) 802.11 specifications addresses the overhead issue by reducing the amount of signaling needed to gain access to a resource. However, while this works well for localized M2M networks, it does not scale-up well as network sizes increase.
Also, conventional random access MAC protocols are not particularly well-suited for cellular networks. Cellular networks are slotted systems; transmissions start and end in the same slot (e.g., a time/frequency slot). This precludes the use of protocols such as CSMA (Carrier Sense Multiple Access) to detect and avoid collisions between UEs attempting to use the same resource. When a collision occurs between UEs, the contested resource goes unused; at the same time, other resources might be available but also go unused. As a consequence, under a protocol such as CSMA, resources are not efficiently utilized. For example, the best channel utilization experienced to date is on the order of 38 percent, achieved using the ALOHA protocol.
Furthermore, it is both difficult and inefficient to allocate resources for M2M random or contention-based accesses because UEs may not be transmitting on a predictable schedule. UEs are typically battery-powered, and hence may be in sleep mode for extended periods of time to conserve power. A resource could be allocated permanently to a UE to ensure that the UE can transmit whenever it is ready, e.g., right after it wakes up. To make efficient use of resources, the permanently allocated resources would have to be small, but that might mean the permanently allocated resources are not large enough to accommodate transmission of all of the UE's data. It would be difficult to determine how large a resource should be in order to ensure low latency and high network efficiency. Also, because there may be more UEs than available resources, some UEs would have to be allocated the same resource. Thus, collisions can still occur if a large number of UEs are awake at the same time.
Therefore, as mentioned above, a protocol that can more efficiently handle M2M traffic would be valuable.